Start with accurate marking out

Every accurate hole begins before the drill touches the metal. If your layout is vague, no amount of careful drilling will rescue the position. Clean the surface first, then apply a layout dye or a smear of marker so your lines show up clearly. Use a scribe and steel rule rather than a pencil, and check datums from the same edge every time so errors do not stack up.

Where a hole must be positioned precisely, scribe two crossing lines and then centre punch the intersection. A light tap with a sharp centre punch gives you a starting cone; a second, firmer tap deepens it without wandering. If the punch mark is off, don't try to drag the drill towards the line — fill the mark with a small dot of weld or ignore it and re-punch. A wandering drill will always follow the deepest cone it finds.

For repeated parts, make a simple drill jig or a drill guide bush in a scrap of steel. It takes twenty minutes to make and saves both scrap and swearing on every subsequent part.

Sharp drills and the right geometry

Blunt tooling is the single biggest cause of oversize, out-of-position holes. A dull drill rubs, work-hardens the surface and pushes the workpiece around. Learn to recognise the signs: a squealing noise, fine powdery swarf instead of chips, and a hole that comes out larger than the drill diameter.

Most general-purpose high-speed steel (HSS) drills come with a 118° point angle, which suits mild steel reasonably well. For harder or tougher materials, a 135° point with a split point (sometimes called a four-facet grind) cuts far more freely and needs less pressure to start. If you drill a lot of stainless, invest in cobalt HSS or a set of split-point bits — you will notice the difference immediately.

Check your drills for damage before use:

• Look along the cutting edges under good light for chips or rounding.
• Check that both flutes are the same length; uneven grinding makes the drill cut oversize.
• Spin the drill in the chuck and watch for visible runout — a bent shank will never drill true.
• Keep a dedicated set for aluminium and brass; the edges last longer and you avoid cross-contamination of swarf.

Sharpen on a bench grinder with a light touch and cool the bit often. If you are not confident freehand, a simple drill sharpening jig pays for itself quickly in saved bits.

Speed, feed and lubrication

Running too fast burns the drill; running too slow rubs it. As a working rule for HSS in mild steel, aim for a surface speed of around 25–30 m/min. Convert that to spindle speed with a simple calculation: RPM equals cutting speed in metres per minute multiplied by 320, divided by the drill diameter in millimetres. A 10 mm drill in mild steel therefore wants roughly 900–950 rpm; a 5 mm drill wants about 1,800 rpm.

Adjust for material:

• Mild steel: around 25–30 m/min.
• Aluminium: 60–120 m/min, with a generous feed to stop the bit rubbing and clogging.
• Brass and bronze: fast, but ease off as the drill breaks through — these grab badly.
• Stainless: 10–15 m/min, plenty of feed pressure, and never let the drill dwell. Rubbing work-hardens the surface and ruins the hole.

Feed matters as much as speed. For drills under 3 mm use around 0.02–0.05 mm per revolution; for 3–10 mm, 0.05–0.15 mm per revolution; above 10 mm, up to 0.3 mm. In practice this means leaning into the cut firmly and steadily rather than pecking timidly. Always use cutting fluid — neat cutting oil for steel and stainless, a light paraffin-based lubricant for aluminium, and nothing but a brush of oil for cast iron, which is best cut dry.

Workholding that will not move

A hole can only be as accurate as the workpiece is secure. Hand-holding is the fastest route to a scrap part and a bruised hand. Use a machine vice bolted to the drill table, or clamp the work down with strap clamps and pack it level so the drill meets the surface square.

Check squareness before you drill. Slide a machinist's square against the quill and the workpiece, or use a dial test indicator on a mill. If the workpiece sits at an angle, the drill will cut oval and drift off line. Where you must drill a curved or angled surface, mill or file a small flat first, then centre punch. For round bar, a V-block keeps the axis true and stops the work spinning.

Pilot holes, step drilling and deep holes

For holes over about 6 mm, start with a small pilot drill, typically 3–4 mm, or use a stubby centre drill for an accurate start. The pilot gives the larger drill a true path and massively reduces wander. If you need a very accurate final size, drill undersize by 0.2–0.3 mm and finish with a reamer.

When depth exceeds roughly three times the drill diameter, withdraw the bit regularly to clear swarf. Packed chips generate heat, dull the edges and can snap the drill. Use a little cutting oil on each retraction, and keep the flutes clear with a brush rather than your fingers.

Check, deburr and learn from each hole

Measure as you go. Calipers or a plug gauge will tell you instantly whether the hole is on size, and a quick check with a square confirms it is straight. If a hole comes out oversize, look first at the drill point, then at the runout, then at your workholding.

Finally, deburr both sides with a countersink or a larger drill turned by hand. A crisp, clean edge is the mark of good work, and it stops a burr from throwing off your measurements on the next operation. Keep a notebook of speeds and feeds that worked for your machines and materials — over a few months it becomes the most useful reference in the workshop.